1. Field of the Invention
The present invention relates to a small-sized current sensor capable of sensing a change in current flowing in a conductor with high sensitivity.
2. Description of the Related Art
One of general methods of measuring control current for controlling control equipment or the like is a method of indirectly measuring control current by detecting the gradient of a current magnetic field generated by the control current. A concrete example is a method of forming a Wheatstone bridge by using four magnetoresistive elements such as giant magnetoresistive elements (hereinbelow, GMR elements) producing giant magnetoresistive effect, disposing the Wheatstone bridge in the current magnetic field, and detecting the gradient of the current magnetic field (refer to, for example, U.S. Pat. No. 5,621,377).
By forming the Wheatstone bridge as described above, noise (interfering magnetic field) from the outside and the influence of the environment temperature can be suppressed to be relatively low. In particular, in the case where characteristics of the four magnetoresistive elements are uniform, a stabler detection characteristic can be obtained.
Another method of further reducing a change in output voltage caused by the environment temperature or noise from the outside by providing a compensating current line is also disclosed (refer to, for example, U.S. Pat. No. 5,933,003).
Recently, because of increasing necessity of detecting weaker current, a magnetoresistive element having a higher impedance and higher sensitivity is being demanded. However, when a Wheatstone bridge is formed by using such a magnetoresistive element having a higher impedance and higher sensitivity, a larger offset is generated and the influence of a variation in connection resistance or the like increases. In many cases, it becomes therefore difficult to adjust the balance of the four magnetoresistive elements.